In comparison to anodic or cathodic electrodeposition (AED or CED, respectively), where the application of electrical voltages is a requirement, electroless corrosion-control coating processes offer the advantage, in particular, of a simpler and less expensive operation and of a shorter operating time. The electroless processes make it possible, in particular, to coat cavities in or edges on the target substrates more effectively than using processes requiring the application of electrical voltages.
In the case of electroless corrosion-control coating, also called ACC (autophoretic chemical coating) process, polymers are generally used, examples being emulsion polymers containing acrylates or styrene/butadiene, which are anionically stabilized. As compared with the aforementioned AED and CED processes, however, the ACC processes have the drawback that the deposited coats exhibit defects which render the substrate significantly more susceptible to corrosion. Consequently, coats of this kind deposited by means of ACC processes are generally treated by rinsing with aqueous coating materials containing chromium, in order to improve corrosion control at the defects. Recently, however, it has turned out that chromium coating materials have great problems in terms of environmental compatibility, and are to be classified as highly hazardous to health. The aim is therefore completely to replace chromium in corrosion-control coatings.
In the train of the development of chromium-free coating materials it has been found, furthermore, that ACC coating materials comprising salts of the lanthanide elements and of the d elements and also an organic, film-forming component likewise ensure very good corrosion control, comparable with that of the chromium coating materials. WO-A-01/86016 describes a corrosion preventative comprising a vanadium component and a further component which comprises at least one metal selected from the group consisting of zirconium, titanium, molybdenum, tungsten, manganese, and cerium. A drawback of corrosion preventatives of WO-A-01/86016 type is the tendency of the metal ions formed from the substrate to migrate through the deposited corrosion-control coat, since the polymers result in deficient film formation.
WO-A-99/29927 describes a chromium-free, aqueous corrosion preventative whose components comprise hexafluoro anions of titanium(IV) and/or zirconium(IV), vanadium ions, transition-metal ions, and phosphoric and/or phosphonic acid. Disadvantages associated with corrosion preventatives of the WO-A-99/29927 type is the tendency of the metal ions formed from the substrate to migrate through the deposited corrosion-control coat, since the polymers result in deficient film formation, and the use of environmentally critical substances, such as hydrofluoric acid or fluorides in particular.
WO-A-96/10461 describes an aqueous corrosion preventative whose components comprise anions with a central atom selected from the group consisting of titanium, zirconium, hafnium, and silicon, and at least 4 fluorine-atom ligands, and an organic polymer dispersion. A drawback of the invention according to WO-A-96/10461 is that deposition of the corrosion preventative on the substrate surface is accompanied by flocculation of the polymer-dispersion particles, which makes their surface contact area small. Moreover, the latex particles have the drawback of a relatively low migration rate in the context of diffusion into cavities or onto edges of three-dimensional substrates, in comparison to polymers whose distribution is molecularly disperse. Moreover, coats with a thickness of between 1 micrometer and 1 mm are formed, entailing a corresponding consumption of material per unit area of the substrate to be coated. Further drawbacks include the tendency of the metal ions formed from the substrate to migrate through the deposited corrosion-control coat, and the use of environmentally critical substances, such as hydrofluoric acid or fluorides in particular.
DE-A-37 27 382 embraces chromium-free, aqueous dispersions of adducts of carboxylic acids and isocyanates with epoxides, which are suitable for autophoretic coating of metallic surfaces. In dispersed form such dispersions have a particle diameter of less than 300 nm, preferably between 100 and 250 nm, and after deposition on the metal surface can be crosslinked at temperatures between 60 and 200° C. Latex particles of this kind, too, have the drawback of having a relatively low migration rate in the context of diffusion into cavities or onto edges of three-dimensional substrates, in comparison to polymers whose distribution is molecularly disperse. Moreover, coats with a thickness of between 1 micrometer and 1 mm are formed, entailing a corresponding consumption of material per unit area of the substrate to be coated and a tendency to form cracks on drying. Further drawbacks include the tendency of the metal ions formed from the substrate to migrate through the deposited corrosion-control coat, and the use of environmentally critical substances, such as hydrofluoric acid or fluorides in particular.
DE-A-103 30 413 describes coating materials which are suitable for coating metallic surfaces and which may comprise caprolactam-modified polyisocyanates based on polyethyleneimines. The coating materials can be applied by deposition coating and, after drying, have thicknesses of between 1 and 300 micrometers. Coats produced in this way likewise require a high level of material and have a tendency to form cracks on drying.
In the light of the aforementioned prior art the problem addressed by the invention was that of finding a corrosion preventative which is largely unobjectionable from an environmental standpoint and which can be applied by a readily technically accomplishable operation to the substrate that is to be protected. In particular the corrosion preventative ought substantially to prevent the migration of the metal ions formed from the substrate and ought to be deposited effectively on edges and in cavities of the substrate. Furthermore, the tendency of the coat to form cracks on drying should in particular be kept as low as possible. Moreover, the effect of extraneous metal ions ought to be kept very low, and effective corrosion control ought to be obtained with a comparatively low level of material employed. In particular, the addition of polar corroding substances, such as particularly corroding salts, to the surface of the metal substrate should be effectively prevented. Furthermore, the conversion coating material ought to develop effective protection for as many different metal substrates as possible and ought to be substantially independent of the redox potential of the substrate to be coated.